1. Field of the Invention
The present invention relates to improvements in a power transmission unit for an electric vehicle, and more particularly to a power transmission unit which has a suitable reduction gear ratio and a high-torque transmission characteristic.
2. Description of the Prior Art
It is well known that a variety of planetary-gear type reducers have been proposed and put into practical use. A planetary-gear type reducer is disclosed on pages 64 to 69 of a publication "Hatsumei (The Invention) VOL. 89". The disclosed planetary-gear type reducer is constituted by two planetary gear sets which have a common sun gear, common pinions and first and second ring gears. The common sun gear is connected to an input shaft, and the second ring gear is connected to an output shaft while the first ring gear is fixed. Such a conventional speed reducer produces a high-reduction gear ratio while keeping a small size. However, this type of speed reducer is not suitable for a reducer for an electric vehicle which requires a reduction gear ratio ranging from 10 to 15 and a high-torque transmitting characteristic.
On the other hand, if such a speed reducer for an electric vehicle is produced by one planetary gear set as shown in FIG. 11, it is necessary to largely decrease the number of teeth of a sun gear 101 and increase a number of teeth of a ring gear 103. This forces an increase in the diameter of this speed reducer. Also, a conventional speed reducer by two planetary gear sets is normally arranged such that an input power is transmitted in turn to an input shaft 110, a first sun gear 111, first pinions 112, a common ring gear 113, a second pinion 114 and an output carrier 115, as shown in FIG. 12. However, this speed reducer necessitates a large number of parts and increases its production cost. Furthermore, this speed reducer becomes large in size by increasing dimensions in diametrical and axial directions.